Microwave-enhanced production of fructose from biomass via glucose isomerization over boron-aluminum-doped hydrochar

Enzymatic glucose isomerization, the benchmark route to fructose, delivers ∼50 % yield under mild conditions yet demands tight pH control, costly cofactors and so on, impeding its deployment in next-generation biorefineries. In this work, boron-species-Al-doped lignin-derived hydrochar (B₁Al₁@HC) was synthesized through hydrothermal carbonization and mechanochemistry modification. The boron group on the catalyst surface adsorbs glucose by forming reversible borate ester bonds with the hydroxyl groups of glucose. Analyses with 27Al nuclear magnetic resonance and density functional theory (DFT) showed that active sites of the catalyst were Al₂O₃ and Al(OH)₃ groups. The as-prepared B₁Al₁@HC catalyst achieved fructose yields of up to 54.8 % from glucose in ethanol at 140 °C for 1 h, exceeding the yield of an enzymatically catalyzed process (∼50%). Activation energy for glucose isomerization over B₁Al₁@HC was 56.9 ± 4.6 kJ/mol, falling well below the values typically reported for Lewis-acid-catalyzed systems. Straw biomass afforded high fructose yields (≥0.24 g/g) upon catalytic conversion. The cost of producing 1 kg of fructose using B₁Al₁@HC is $47.6, lower than the laboratory free glucose isomerase, and the life cycle assessment results show that the carbon emission of its production process is 4.26 kg CO₂ eq. The catalytic system allows efficient conversion of glucose to fructose due to coordinated interactions between glucose and Al and boron groups.